So-called multi-media industry has been developed and broad band communication has been increasingly demanded, so that it has been already applied an optical communication system operating at a speed higher than 10 Gb/s. Such system operating at even higher speed is expected. An LN optical modulator is applied as a device for modulating electrical signal of 10 Gb/s or higher (microwave signal) to light.
It was invented a structure to satisfy velocity matching condition of microwave and light wave to realize wide-band modulation of an optical modulator, by lowering the thickness of an optical waveguide substrate (Japanese Patent publication No. 2002-169133A).